An entire branch of the science of hybrid microelectronics is based upon the use of thin film materials deposited in vacuum on insulating substrates. Surfaces of this type are generally constructed by the deposition of separate conductive and resistive films, with desired patterns thereafter being generated by the use of photo-lithographic techniques, e.g. by selective photo-etching of the various layers so as to achieve the desired patterns of resistors and conductors.
A class of materials often employed as a resistive material, are the nickel-chromium alloys, e.g. those compositions commonly known to those skilled in the art, under the trademark "Nichrome". These materials have a long history of utilization in thin film hybrid circuitry. They are most noteworthy in exhibiting a low temperature coefficient of resistivity (TCR); and under most conditions of operation, very high stability. A most serious drawback however is their vulnerability to corrosive degradation under certain environmental circumstances; e.g. under certain conditions, such as in the presence of moisture and an electrical bias, the thin nickel-chromium (hereinafter sometimes referred to as Ni--Cr) films can be impaired by electrolytic attack.
From time to time proposals have been made for armoring the Ni--Cr thin films to achieve supplemental protection against environmental effects. For example, in Alger U.S. Pat. No. 3,112,222, an overcoating of titanium oxide is provided for such purposes. In practice, however, it is found that protective layers of materials such as silicon monoxide or the titanium oxide mentioned, are less than fully effective, particularly when exposed to moisture. In particular this type of protective layer tends to be relatively lacking in continuity by virtue of pin holes and other undesirable inclusions. Furthermore, it is necessary to in some manner effect electrical connection to the Ni--Cr layer, in order to define a resistive path between conductor terminals; and difficulties are commonly encountered in effecting such connections where the nature of the armor layer is not physically and electrically compatible with the nickel-chromium alloy.
The material tantalum is widely known for use in resistors in thin film microcircuits application, particularly in the form of tantalum nitride. Thin films of this type are characterized in general by their overall stability and reliability and in particular by an exceptional ability to successfully withstand corrosive attack. While the environmental characteristics of the tantalum-based materials are therefore excellent, the materials have been deemed less than ideal for use as resistive elements, primarily because of their relatively high TCR. Whereas nickel-chromium films can be readily prepared with a TCR of less than .+-.25 ppm/C.degree., tantalum nitride is difficult to fabricate with a TCR of less than 100 ppm/C.degree..
It has also been proposed in the art, to fabricate thin film resistors in the form of multi-layer structures, wherein the various layers each serve a resistive function, i.e. as opposed to the instance where an upper layer is protective in nature. For example, in U.S. Pat. No. 3,864,825, a thin film resistor structure is disclosed, wherein layers of resistive materials are superimposed upon an area of the substrate. The said patent teaches that the materials of highest resistivity be deposited adjacent to the substrate, with the remaining layers being in the order of decreasing resistivity. It is shown in this patent, that by such an arrangement one may selectively remove one or more of the various layers, so that the equivalent resistance of the remaining layers has a specific value of interest for a given application.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a composite thin film resistor structure, which displays the desirable thermal stability characteristics of the nickel-chromium alloys, while yet being exceedingly resistant to environmental attack.
It is a further object of the present invention to provide a composite thin film resistor, and method for manufacture thereof, wherein the advantages of the nickel-chromium thin films with respect to the thermal stability properties, are combined with the environmental-resisting characteristics of tantalum-derived thin films.